Commas

Use commas sparingly, only when breaks are needed for readability
and to avoid confusion or misinterpretation. APS style requires the use of the
series comma. When in doubt, follow the guidelines in CBE, 6th ed., and The
Chicago Manual of Style.

When to use a comma:

To separate the elements (words, phrases, or clauses) of a simple series of more than two. If any of the elements contain internal puctuation, separate them with semicolons.

To separate two independent clauses joined by a coordinating conjunction. CBE makes the comma optional if the clauses are not long or complex. Follow
the author's lead in a manuscript unless the comma is needed for clarity.

To set off an introductory clause beginning with a subordinating conjunction (if, although, because, when,
since). CBE makes it optional if the clause is short; however, that is unusual in scientific writing. If the subordinate clause falls at the end of the sentence, follow
the manuscript for use of the comma before the subordinating conjuction.

To set off an introductory infinitive phrase ("To accomplish the task,
we...") or participial phrase ("Using hippocampal slices from adult rats, we studied...")
Beware of dangling participles; it is a mistake that
authors make frequently! In the previous example, you would not change
"using" to "with" or "with the use of."

To set off a single introductory prepositional phrase to prevent misreading. A comma after a
short introductory prepositional phrase is optional. Always insert a comma after a series of two
or more introductory prepositional phrases.

In all, eight experiments were performed.

After three periods of eight hours each, the rats were
removed to...

To separate a nonrestrictive phrase or clause from the rest of the sentence.
A nonrestrictive clause begins with "which." Frequently authors use "which" for both
restrictive and nonrestrictive clauses; change "which" to "that"
or vice versa when required.

good

Rats were fasted for 4 h before each experiment, which was sufficient
time to establish a steady state.

The animals, which were fasted overnight, were then injected with
50 ml of...

bad

The high prevalence of antibodies to the Bartonella species which
were examined in this study indicates a possibility of infection...

To separate a nonrestrictive appositive from the rest of the sentence.

Raymond Turner, a biologist, described the species.

To set off a conjunctive adverb (therefore, thus, still, however, accordingly, moreover, nevertheless, consequently) or a transitional phrase (by comparison, on the other hand, in fact) that introduces a distinct break in the continuity of
thought (an interjected or parenthetical phrase). If the author has not used commas, do not insert.

To set off "i.e." ("that is") and "e.g." ("for
example"), whether abbreviated or expanded; to set off "respectively", which usually falls at the end of the sentence.

Before injection of picrotoxin, the cell responded in its normal way, i.e.,
most strongly to ipsilateral, small-field motion from front to back.

The longest delays included in the calculations were 14, 9, and 4 ms for
the FM-FM, DF, and VF areas, respectively.

After multiple adjectives modifying the same noun.

monosynaptic, excitatory linkage

binocular, optokinetic stimulation

Inside double quotation marks, even when the comma is not part of the quoted material.

Inside superior letters or numbers used as footnotes.

To set off contrasted or antithetical phrases or clauses.

The greater the risks, the more gratifying will be the
results.

To set off words, phrases, or clauses placed out of their natural order for emphasis or clarity.

To set off adjacent numbers.

In 1908, 45 experiments were...

When not to use a comma:

Before the word "and" when it connects two parallel elements, e.g., subjects, verbs, phrases,
or dependent clauses. This is a frequent comma error in both foreign and US papers.

Dopamine is released from carotid body glomus cells and
chemoafferent fibers in the nucleus tractus solitarii.

Before or after a one-word appositive.

The GABAA receptor agonist muscimol did not influence the response of
WDR neurons in normal or allodynic animals.

With a restrictive adjective clause or phrase. (A restrictive clause begins with
"that," although many authors use "which" for both restrictive and nonrestrictive
modifiers.) As a general rule, do not add commas before and after
prepositional phrases (making them nonrestrictive).

good

The occurrence of different mechanisms that
depend on the degree of hypoxia could explain these differences.

bad

Results with knockout mice are most clearly interpreted in
experimental paradigms, that take place over days to weeks.

To separate coordinate clauses joined by a conjunctive adverb (e.g., "however") rather
than a coordinating conjunction. Use a semicolon instead.

It was not possible to apply parametric trend tests to these data; however,
a Wilcoxon's test revealed that overall the discharge rate was significantly
higher after the injury.

After the abbreviation "cf." ("compare").

After "thus" or "therefore" unless followed by a subsidiary
clause or phrase:

Thus activation threshold gave no evidence for major contributions from nonsoma sites.